The present invention relates to a modulated signal generation apparatus for supplying a modulated test signal to various electronic devices generally incorporated in the digital communication system such as an automobile telephone system, a portable telephone system, and a simple portable telephone system and, more particularly, to a modulated signal generation apparatus incorporating a fading simulator.
As is well known, in the digital communication system, a plurality of mobile stations 2 can communicate with one base station or cell site 1, as shown in FIG. 7.
Signals exchanged between the base station 1 and the mobile stations 2 are generally modulated signals prepared by modulating radio-frequency carrier signals by transmission data.
When the performance of each electronic device incorporated in the digital communication system is to be measured, a modulated test signal a is generated by a modulated signal generation device 3 and supplied to a device under test 4, and an output signal b from the device under test 4 is input to a testing equipment 5, as shown in FIG. 8A.
The output signal b is analyzed by the testing equipment 5 to measure various performances of the device under test 4.
When each mobile station 2 is connected to the base station 1 using a radio medium as in the digital communication system shown in FIG. 7, a fading phenomenon as a phenomenon unique to the radio wave occurs in transmitted and received signals.
Accordingly, the fading phenomenon also occurs in signals input to the base station 1 and each mobile station 2.
For this reason, the antifading performances (the reciever performances in fading enrichment) of various electronic devices of the base station 1 and each mobile station 2 must be measured.
To measure the antifading performance, a fading simulator 6 is inserted between the modulated signal generation device 3 and the device under test 4 to measure the antifading performance, as shown in FIG. 8B.
More specifically, the fading simulator 6 is used to forcibly cause fading in the modulated test signal a output from the modulated signal generation device 3, supplying a modulated test signal a.sub.1 containing fading to the device under test 4.
The testing equipment 5 analyzes an output signal b.sub.1 from the device under test 4 to measure the antifading performance of the device under test 4.
In this case, the fading simulator 6 is constituted as shown in FIG. 9.
The modulated signal generation device 3 outputs an analog modulated test signal a quadrature-modulated by a plurality of data.
If t is the time, and j is the imaginary unit, then the modulated test signal a is given by an equation of adding in-phase and quadrature components to each other such as equation (1): EQU r(t)=Re{r(t)}+j.multidot.Im{r(t)} (1)
The modulated test signal a input to the fading simulator 6 is input to a quadrature demodulator 7.
The quadrature demodulator 7 demodulates the modulated test signal a into a baseband signal made up of an in-phase component signal Re{r(t)} and a quadrature component signal Im{r(t)} by using a carrier frequency f.sub.c of the modulated test signal a.
The in-phase component signal Re{r(t)} and quadrature component signal Im{r(t)} of the baseband signal are respectively input to multipliers 8a, 8b, 9a, and 9b.
Noise generators 10a and 10b generate white Gaussian noise signals.
The white Gaussian noise signals output from the noise generators 10a and 10b are respectively frequency-regulated by fading filters 11a and 11b on the output stage to be output as fading signals.
That is, the white Gaussian noise is frequency-regulated by the fading filter having pre-equalizing frequency characteristics to be changed into colored noise with a very narrow band. This chromatic noise has been proved to extremely approximate the fading phenomenon.
A fading signal CN(t) generated by the noise generators 10a and 10b and the fading filters 11a and 11b is given by an equation of adding the in-phase fading signal Re{CN(t)} and the quadrature fading signal Im{CN(t)} to each other such as equation (2): EQU CN(t)=Re{CN(t)}+j.multidot.Im{CN(t)} (2)
One fading filter 11a outputs the in-phase fading signal Re{CN(t)}, whereas the other fading filter 11b outputs the quadrature fading signal Im{CN(t)}.
The in-phase fading signal Re{CN(t)} is supplied to the multipliers 8a and 9a, and the quadrature fading signal Im{CN(t)} is supplied to the multipliers 8b and 9b.
Output signals from the multipliers 8a and 9b are added by an adder 12a, and the obtained signal is input as a new in-phase component signal Re{S(t)} to one terminal of a quadrature modulator 13.
Output signals from the multipliers 9a and 8b are added by an adder 12b, and the obtained signal is input as a new quadrature component signal Im{S(t)} to the other terminal of the quadrature modulator 13.
The new in-phase and quadrature component signals Re{S(t)} and Im{S(t)} which have undergone fading and are input to the quadrature modulator 13 are given by equations (3) and (4): EQU Re{S(t)}=[Re{r(t)}.times.Re{CN(t)}]-[Im{r(t)}.times.Im{CN(t)}](3) EQU Im{S(t)}=[Re{r(t)}.times.Im{CN(t)}]+[Im{r(t)}.times.Re{CN(t)}](4)
The quadrature modulator 13 quadrature-modulates a carrier signal having the frequency f.sub.c with the baseband signals of the in-phase and quadrature component signals Re{S(t)} and Im{S(t)}, and externally outputs the quadrature-modulated signal as a new modulated test signal a.sub.1.
The modulated test signal a.sub.1 which has undergone fading and is output from the fading simulator 6 is input to the device under test 4.
An amplitude disturbance factor A(t) in the modulated test signal a.sub.1 having undergone fading is given by equation (5): EQU A(t)=[Re{CN(t)}.sup.2 +Im{CN(t)}.sup.2 ].sup.1/2 (5)
A phase disturbance factor .phi.(t) in the modulated test signal a.sub.1 is given by equation (6): EQU .phi.(t)=tan.sup.-1 [Im{CN(t)}/Re{CN(t)}] (6)
The device under test 4 receives the modulated test signal a.sub.1 having undergone fading in which the amplitude and the phase change in accordance with equations (5) and (6).
The testing equipment 5 analyzes the output signal b.sub.1 from the device under test 4 to measure the antifading characteristics in the device under test 4.
However, the fading simulator 6 shown in FIG. 9 still suffers the following problems which should be solved.
Since noise signals generated by the noise generators 10a and 10b are pure white Gaussian noise signals, they are stochastic process signals in both the amplitude and phase directions.
Both the amplitude and phase characteristics of the fading signal CN(t) prepared by the noise generators 10a and 10b and the fading filters 11a and 11b therefore change within only a predetermined range determined by the statistical amount.
In other words, the amplitude and phase characteristics of the modulated test signal a.sub.1 output from the fading simulator 6 shown in FIG. 9 irregularly change within only the predetermined range.
For this reason, the fading simulator 6 shown in FIG. 9 cannot generate the modulated test signal a.sub.1 in which a fading phenomenon having arbitrary amplitude and phase characteristics conforming to practical use conditions has occurred.
In the fading simulator 6, as shown in FIG. 9, the radio-frequency modulated test signal a generated by the modulated signal generation device 3 is demodulated into an original baseband signal by the quadrature demodulator 7. This baseband signal is subjected to fading addition signal processing such as multiplication and addition with a fading signal, and then quadrature-modulated by the quadrature modulator 13 again to obtain the radio-frequency modulated test signal a.sub.1 added with fading.
Amplitude balance mismatching, phase balance mismatching, a frequency error, or the like caused by the quadrature demodulator 7 occurs in the modulated test signal a.sub.1.
As a result, a disturbance other than a fading characteristic parameter is mixed in the fading-added modulated test signal a.sub.1 supplied to the device under test 4.
When the testing equipment 5 measures the antifading characteristics of the device under test 4 by comparing the modulated test signal a output from the modulated signal generation device 3 and the output signal b.sub.1 output from the device under test 4 with each other, accurate antifading characteristics cannot be obtained due to the presence of a disturbance parameter other than fading.